Dietary factors play an important role in the etiology of breast cancer. Vegetables of the Brassica genus, such as broccoli and cabbage, contain isothiocyanates (ITC) which increase glutathione-S-transferase (GST) activity, leading to the excretion of potentially carcinogenic compounds. Brassica vegetable administration prevented mammary tumor development in animal models of breast cancer, and it is therefore conceivable that Brassica consumption could reduce breast cancer risk in humans. Preliminary results from our pilot study suggest that high levels of urinary ITC excretion, indicative of greater Brassica vegetable intake, were associated with a greater than 50% reduction in breast cancer risk. We propose to analyze urine samples collected from a larger subset (n=350 case-control pairs) of study participants recruited into the Shanghai Breast Cancer Study, a NCI-funded population-based case- control study among Chinese women in Shanghai (RF01 CA64271). In addition to in-person interviews, fasting blood and urine samples have been collected from over 80% of the 3000 women included in this parent study. These samples are being used for several ancillary studies, including NCI-funded studies to evaluate the relation of estrogens, IGFs, pesticides, genetic factors, and phytoestrogens, with breast cancer risk (R03CA80655, R03CA83050, R03CA86119, NCI contract). For this newly proposed individual matched case-control study, urinary ITC levels will be analyzed by HPLC, and GST genotype determined from blood DNA. Because recruitment, questionnaire data, and specimen collection have been completed by existing studies, this project will be very cost-efficient. Urinary ITC excretion predicts habitual Brassica intake within Asian populations, as people in China or Japan typically consume 200 g/day/person of Brassica. Since Brassica vegetables are widely available and without toxicity, a protective association between urinary ITC levels and breast cancer could suggest that dietary recommendations to reduce breast cancer risk should include greater Brassica consumption.